Impatiens downy mildew: Gardeners need to know

April 17, 2013

In recent weeks, several alerts focusing on impatiens downy mildew (Plasmopara obducens), an oomycete, or water mold disease, which is related to the pathogen commonly referred to as late blight of tomatoes and potatoes, have been circulating among Extension educators across the northeast. Fortunately, impatiens downy mildew strikes garden impatiens, (Impatiens walleriana) and little else.

IDM is an extremely destructive foliar disease, defined by a somewhat velvety growth of fine white spores, visible only on the underside of infected leaves, which often curl under, turn yellow and drop from the plant. It can take as long as two weeks for the spores to become visible, so plants can appear healthy, even after they have become diseased. Flowers will drop, too, until eventually the flourishing mounds of rich green foliage and colorful blossoms have been reduced to small bunches of anemic flower buds and sickly yellow leaves hanging at the tips of miserable-looking, unadorned stems. Sooner or later, the plant dies.

In the United States, IDM has existed in the wild since the late 1800s but, until this century, because it had never been seen in production markets, it had been considered more or less non-threatening. During the last decade, however, IDM has somehow transformed into a devastatingly aggressive and widespread bedding plant disease.

The disease first appeared in England in 2003, with no prior history in that country. The following year, outbreaks were observed in production greenhouses in the U.S. Those occurrences were few and irregular but, nonetheless, raised the possibility that the disease had arrived here from England. According to Margery Daughtrey, a plant pathologist at Cornell's Long Island Horticultural Research and Extension Center in Riverhead however, "Nobody knows that."

In 2008, occurrences were confirmed in landscapes, as well as production operations in several eastern states. Reports continued to surface in 2009 and 2010, but remained sporadic until 2011, when outbreaks were unexpectedly verified in landscape and container plantings in California, Illinois, Indiana, Minnesota, Massachusetts and New York. Reports from Wisconsin and Ontario, Canada could not be authenticated but, because most gardeners and landscapers are unacquainted with IDM, there was wide-ranging belief that existence of the disease was substantially greater within the landscape than reports had indicated.

Last year, that thinking may have been validated. Outbreaks of IDM suddenly became extraordinarily widespread. Plant pathologists confirmed its existence in 33 states, including Washington and Oregon on the West Coast, Michigan, Wisconsin, Indiana, Illinois, Tennessee, Kentucky and others in the Midwest, every state on the Gulf Coast from Texas to Florida, and every state on the Eastern Seaboard, from Florida to Maine, including the District of Columbia. The disease has also been confirmed in other European countries, Australia and South Africa.

Because some downy mildews can be transmitted through seed, it is important to note here that there is currently no evidence of seedborne transmission of IDM. Unfortunately, however, the IDM pathogen is able to survive the winter in soil. So, if you had impatiens affected with IDM last year, it is almost certain that inoculum living in your garden soil will infect healthy plants introduced into those garden beds this year. Alas, it will most likely be years before you can safely plant walleriana varieties of impatiens in your garden again. What's more, even if you did not see the disease last year, the safety of impatiens planted in your garden this year is not assured. Winds can carry the spores great distances; by some accounts, more than a hundred miles.

For those of us who have come to rely on impatiens because of their proven performance in bringing an impressive variety of appealing color to shaded garden settings, this knowledge is extremely disheartening. Impatiens are a time-honored choice for gardeners of every ilk; allowing green thumbs and greenhorns alike the ability to create blankets of striking shade-garden color that lasts all season long.

Because of this, impatiens have become one of the most important bedding plants the industry has to offer. According to the U.S. Department of Agriculture's National Agricultural Statistics Service, the wholesale value of impatiens sold last year, in New York alone, was nearly $1.5 million. The effects these outbreaks will have on the industry are yet to be seen.

The good news is this. Although they may host other downy mildews, no other bedding plants are reported to be hosts of IDM. Margery Doughtrey suggests planting shade-loving varieties of coleus, salvia, browallia, caladiums and torenia, sometimes called wishbone flower, instead. And, since IDM is so very host-specific, Doughtrey also recommends that home gardeners consider varieties of New Guinea and Fanfare impatiens, as well as those sold under the registered trademark name Sunpatiens. All are, it is believed, not susceptible to IDM. Keep in mind, however, that all these newer varieties of hybrid impatiens, although widely recognized as reasonable performers in partial shade, are nevertheless, bred for sun.